Trough waveguide antennas



Dec. 26., 1961 w. ROTMAN TROUGH WAVEGUIDE ANTENNAS 2 Sheets-Sheet 1 Filed March 20, 1957 INVENTOR. M14727? Raf/IA ill/fill I MM, A46 147' 7' dk/VE Pf Dec. 26, 1961 w. ROTMAN TROUGH WAVEGUIDE ANTENNAS 2 Sheets-Sheet 2 Filed March 20, 1957 INVENTOR. IVA-47E)? Edi/f4 Patented Dec. 26, 1951 3,015,100 TROUGH WAVEGUIDE ANTENNAS Walter l totmau, 8 Chiswick Road, Brighton, Mass. Filed Mar. 20, 1957, Ser. No. 647,454 Claims. (Cl. 343-=772) (Granted under Title 35, US. Code (1%2), sec. 266) The invention described herein may be manufactured and used by or for the United States Government for governmental purposes without payment to me of any royalty thereon.

This invention relates generally to antennas and more particularly to apparatus for the transmission of highfrequency electrical energy. The invention is characterized by the utilization of a special waveguide structure as a means for radiating energy to the surrounding space and involves the use of elements within the waveguide structure for controlling the guide wavelength or phase velocity and/ or radiation intensity or attenuation rate in the waveguide. Control of these parameters in efiect enables the obtaining of any desired controlled radiation pattern.

A trough waveguide of either rectangular channel shape, U-shape, V-shape or variations thereof having a symmetrically disposed fin therein comprises the transmission line which is modified to produce the controlled radiation. The basic concept on which this invention relies is that anti-symmetrical obstacles in a trough waveguide couple energy from the bound symmetrical trough guide mode, a transverse electric (TE) mode, into energy in a transverse electromagnetic (TEM) field which radiates into free space from the open side of the Waveguide. Syrnetrical obstacles, on the other hand, react as tuning elements. In accordance with above principles, a large variety of radiating devices may be constructed.

The trough waveguides, together with their radiating elements, have utility alone or as primary line source arrays for use with secondary reflectors, horns, or lenses in radar and communication antenna systems. In this respect, they are superior to slots or dipole equivalent arrays on rectangular or circular waveguides.

Because of the superior electrical and mechanical properties of the antennas of this invention, many new advantages are realized.

It is contemplated that the subject invention will open new vistas in the fields of radio astronomy, radar, and communications.

It is an object of this invention to produce an antenna comprising a trough waveguide with means for controlling radiation therefrom.

It is another object of the invention to produce an antenna with means for controlling guide wavelength.

It is still another object of the invention to produce an antenna of extremely broad impedance bandwidth.

It is a further object of the invention to produce an antenna which may be composed of either resonant or nonresonant radiating elements.

It is a still further object of the invention to produce a non-resonant traveling wave array composed of nonresonant elements.

It is another object of the invention to produce an antenna which is easily and economically produced by conventional, commercial manufacturing techniques.

it is still another object of the invention to produce a novel resonant radiating element for use in a trough waveguide.

A further object of the invention involves the production of a novel traveling wave antenna array of resonant radiating elements.

A still further object of the invention involves the production of a traveling-wave leaky-waveguide antenna.

Another object of the invention involves the production of a traveling wave array of non-resonant radiating elements in which there is achieved a phase reversal between successive elements.

Still another object of the invention involves the production of a traveling wave leaky-waveguide formed by a continuous asymmetry along a trough waveguide.

further object of the invention involves the production of a novel end fire or near end fire radiation antenna.

These and other advantages, features and objects of the invention will become more apparent from the following description taken in connection with the illustrative embodiments in the accompanying drawings, wherein:

FIGURE 1 is a cross-sectional view of a trough waveguide with a representation of the electric field of the transverse electric (TE) mode therein;

FIGURE 2 is a cross-section of a trough waveguide with a representation of the electric components of the transverse electromagnetic TEM) field;

FIGURE 3 is a cross-section of a trough waveguide with a TE mode to TEM field coupler;

FIGURE 4 is a cross-sectional view of a trough waveguide with a tuning element for the TE mode;

FIGURE 5 is a pictorial view of a trough waveguide with an L-rod radiator;

FIGURE 6 is a pictorial representation of an antenna with non-resonant discrete radiating elements;

FIGURE 7 shows a traveling wave array of capacitive rods in a trough waveguide;

FIGURES 8-11 are representations of a trough waveguide with asymmetrical arrangements to produce radiation; and

FIGURE 12 is a top view of a trough waveguide antenna for broadside radiation.

The invention utilizes a symmetrical non-radiating trough waveguide which acts as a transmission line. This type of waveguide in its mechanical construction is left open on one side; however, it still acts as a transmission line. Its characteristics, frequency-wise follow those of a waveguide while it retains mechanical simplicity of a strip transmission line. The general configuration of the waveguide has two side walls and a bottom, thus forming a trough, and a substantially centrally disposed fin member of less height than the side walls. The embodiment of FIGURE 1 is a channel or rectangular form of trough waveguide but various alternative forms can be achieved by deformation of the side walls, for example, the trough may be of U or V shape as long as symmetry about the central fin is maintained in order to avoid spurious modes.

Like numerals will designate like parts of the wave guide throughout the specification.

FIGURE 1, for illustration purposes, shows a rectangular trough waveguide having side walls 20 and 2t, a bottom wall 22 and a substantially centrally disposed fin member 23. When the side walls 20 and 21 are less than one half wavelength apart, a TE (transverse electric) mode may be propagated along the axis of the guide. This mode is bound to the center fin and has an electric field with a general configuration as shown in FIGURE 1. The intensity of the field lines of the electric vector increases from the bottom 22 of the waveguide to the top of the central vane or fin 23. The transverse currents on the sides of tin 23 vary from a minimum at the free edge to a maximum at its base.

The electric components of the TEM (transverse electromagnetic) field is depicted in FIGURE 2 and can be propagated wherever side walls exist. As is shown by the dashed lines at the open end of the trough, radiation into space is achieved whenever the field strikes the open top or aperture of the guide. The creation of an asymmetry causes energy from the TE field to be converted into a TEM field which is not bound to the center fin. The TEM fields are capable of being propagated in all directions at right angles to the TE vectors thereby allowing a release of energy through the open side of the waveguides.

Although the trough waveguide is shown as a unitary structure of highly conductive material, each of portions designated as 29, 21, 22 and 23, could be made of separate pieces of stock of any material secured together by conventional means as long as the interior of the trough is plated or otherwise lined with a highly conductive material.

The characteristics of a trough waveguide are such that the cut-off wavelength depends upon the electrical height of the center fin 23, i.e., the cut-off Wavelength is approximately that at which the centcrfin is a quarter wavelength. The side wall (20, 21) height above the center fin 23 and the spacing between side walls act to prevent unwanted, uncontrolled radiation. Less than half wavelength spacing between the side walls allows for operation of the line over a range of frequencies; The spacing of the side walls, at any rate, should not exceed a half wavelength of the highest frequency in the range. The TE mode is critically dependent upon the dimensions of the center fin While the TEM or radiating mode is independent of the fin 23. 7

FIGURE 3 represents a basic principle or concept upon which this invention is based. An anti-symmetrical obstacle, such as a horizontal rod 24, has the ability in a trough waveguide to convert some of the energy from the bound TE mode into the T EM field which is then radiated into free space. Utilization of this principle allows for the production of controlled radiation along the length of the waveguide. The rod 24, perpendicular to center fin 23, creates an asymmetry such that when rod 24 is excited, it reradiates the incident energy in both the TEM field and the TE mode thereby causing a portion of the incident energy from the TE mode to be converted into the TEM field while another portion is reflected back in the TE mode towards the source. The shunt admittance of the rod as measured in the trough guide appears to have two components: a reactive susceptance determined by the amount of reflected energy and a conductance governed by the amount of energy coupled between the TB mode and TEM field and subsequently radiated. The rod 24 will appear as a resonant element if the capacitive susceptance is cancelled by an additional reflection, equal in magnitude but opposite in phase. Changing the length of the rod increases both the susceptive and conductive components. It should be understood that the rod may be projected from the side wall to achieve the same eiiect.

Symmetrical obstacles do not couple the TE mode and TEh l field; therefore, they may be used as tuning elements. The vertical post 25 placed above fin 23 (FIG- URE 4) is greater than a quarter wavelength and by virtue of its symmetry does not couple energy, but causes a reflection which appears as an inductive shunt susceptance.

The antenna of FEGURE 5 utilizes the principles brought out relative to FIGURES 3 and 4. By combining the horizontal rod 24 and vertical rod 25 into an L-shaped member 26, the horizontal asymmetrical rod, being less than a quarter wavelength, is capacitive in nature while the vertical rod, between one quarter and one half wavelength long, provides proper inductive compensation. Linear arrays either resonant or non-resonant depending on its termination may be constructed from a series of resonant discrete L-rod elements 26 by properly orienting and spacing them along the trough waveguide to control phase and by varying the lengths of the rods to control amplitude. By having the horizontal rods on alternate sides of the fin or on the alternate side walls, 189 phase reversal is achieved.

The traveling wave antenna ofFIGURE 6 utilizes nonresonant discrete radiating elements comprising a combination of series and shunt reactive elements as representedby the notches 27 which act as series inductances K" and the relatively short post 28 which acts as a capacitive shunt and an asymmetric radiating rod 29. The combina tion may be repeated at half wavelength intervals for the purpose of constructing an array.- The antenna has the advantage that a matched load at the output sides looks like a matched load at the input side.

Under certain conditions, for maximum radiation, addi-' tional notches and/o1: posts may be required. Radiating rod 29 may be placed on post 28 or on the fin 23 and,.

since an asymmetry is produced, radiation will occur, the intensity of which is governed by the length of the rod and its height from the base 23. introduced in an array by placing the radiating element on alternate sides of central fin 23 at half wavelengthintervals.

A traveling wave, non-resonant antenna composed of non-resonant capacitive elements may also be constructed in accordance with this invention.

a series of closely spaced horizontal rods 30, asymmetrically located along the center fin 23. The phase velocity of the traveling wave, modified by the presence of the horizontal rods, is controlled by adjusting the height of the center fin While its rate of attenuation is determined chiefly by the length of the rods; therefore, a particular array may have various lengths of rods and a varying height for the tin.

When the array is continuous the device acts as a leaky waveguide and end fire radiation results.

By projecting capacitive rods 3% on alternate sides of the fin for half wavelength intervals, broadside radiation; may be obtained. 7

A non-resonant traveling wave linear array can be obtained by continuous asymmetry in the trough waveguide since the TB mode cannot exist completely bound in an asymmetrical structure and gradually converts to the TEM field as it progresses along the guide. The rate of conversion depends on the degree and nature of the asymmetry.

FIGURES 8-11, disclose various asymmetric nonresonant leaky waveguide antenna arrays. In FIGURE 8, the radiation field 31 is shown with a trough waveguide wherein one side is filled with a metallic conductor 32 which, as illustrated, includes the center fin. FIGURE 9 illustrates a continuously bent center fin 23 to produce the asymmetry while FIGURES 10 and 11 have one side of the waveguide partially filled with either a conductor 33 or dielectric 3d. The attenuation per unit length and phase velocity may be calculated or determined experimentally for the various types of material and efiective heights which create the asymmetry such that almost any desired characteristic may be obtained. As shown, the metallic filled portion of FIGURES 8 and 10 appear to be machined as part of the trough; however, it should be understood that the trough may be made of separate parts for each of the sides and fin while the metallic filling material may be a separate block to produce the desired configuration or may be cast in position. With a metal conductor only on one side of the trough, the radiation is between end fire and broadside, i.e., it radiates in the forward quadrant of space. Dielectric materials on the other hand, having less radiation per unit length, slows down the propagated wave such that end fire radiation is possible. Combinations of dielectric with metal give a resultant radiation characteristic containing the combination of the characteristics of the individual materials.

FIGURE 12 illustrates an embodiment showing utilization of the principle of phase reversal with asymmetric period obstacles and tuning elements to produce an antenna with broadside radiation. Flanges 35 (only one of which is shown) form a horn to improve the efiiciency of the radiation from the trough waveguide.

Blocks or obstacles 36 which maybe of any shape in longitudinal cross section, or have various heights or thicknesses depending on "the desired radiation properties are Phase reversal may be In FIGURE 7, the vertical tuning elements have been eliminated leaving only spaced for broadside radiation at half wavelength intervals in a staggered relationship on alternate sides of fin 23 to produce a phase reversal of 180 between successive obstacles to the traveling wave. Posts 37 comprise tuning means to cancel reflections and eliminate impedance mismatch.

Since all the radiating points are in phase and spaced closer than the wavelength in free space, radiation would be broadside or near broadside to the array.

Although the invention has been described with reference to particular embodiments, it will be understood to those skilled in the art that the invention is capable of a variety of alternative embodiments within the spirit and scope of the appended claims.

I claim:

1. A waveguide adapted for the interchange of energy between itself and surrounding space, said waveguide comprising a transmission line having two parallel side walls connected by a bottom wall, said side walls being completely open at the upper edges thereof, thus forming a structure of a trough-shaped cross section which is substantially constant throughout its length, a center fin symmetrically disposed within said generally troughshaped structure and running the length thereof, means for producing and controlling radiation from the open top of said trough waveguide structure, said means being so located as to cause an asymmetry with respect to a lateral cross section of said waveguide, and means in said waveguide for cancelling reflections caused by said radiation producing means.

2. A waveguide adapted for the interchange of energy between itself and surrounding space, said waveguide comprising a transmission line having two parallel side Walls connected by a bottom Wall, said side wall being completely open at the upper edges thereof, thus forming a structure of a trough-shaped cross section which is substantially constant throughout its length, a center fin symmetrically disposed within said generally troughshaped structure and running the length thereof, means for producing and controlling radiation from the open top of said trough waveguide structure, said means being so located as to cause an asymmetry with respect to a lateral cross section of said waveguide, and means in said waveguide for cancelling reflections caused by said radiation producing means, said reflection cancelling means being symmetrically located with respect to a lateral cross section of said waveguide.

3. A waveguide adapted for the interchange of energ between itself and surrounding space, said waveguide comprising a transmission line having two parallel side Walls connected by a bottom wall, said side walls being completely open at the upper edges thereof, thus forming a structure of a trough-shaped cross section which is substantially constant throughout its length, a center fin symmetrically disposed within said generally troughshaped structure and running the length thereof, and means for producing and controlling radiation from the open top of said trough waveguide structure, said means being so located as to cause an asymmetry with respect to a lateral cross section of said waveguide, said asymd metrically located means comprising a discrete element mounted on the interior wall of said trough waveguide.

4. A waveguide adapted for the interchange of energy between itself and surrounding space, said waveguide comprising a transmission line having two parallel side walls connected by a bottom wall, said side Walls being completely open at the upper edges thereof, thus forming a structure of a trough-shaped cross section which is substantially constant throughout its length, a center fin symmetrically disposed within said generally troughshaped structure and running the length thereof, means for producing and controlling radiation from the open top of said trough waveguide structure, said mean being so located as to cause an asymmetry with respect to a lateral cross section of said waveguide, and series and shunt reactive elements superimposed on said center fin for cancelling reflections introduced by said asymmetrically located means.

5. A waveguide adapted for the interchange of energy between itself and surrounding space, said waveguide comprising a transmission line having two parallel side walls connected by a bottom wall, said side walls being completely open at the upper edges thereof, thus forming a structure of a trough-shaped cross section which is substantially constant throughout its length, a center fin symmetrically disposed within said generally troughshaped structure and running the length thereof, means for producing and controlling radiation from the open top of said trough waveguide structure, said means being so located as to cause an asymmetry with respect to a lateral cross section of said waveguide, and series and shunt reactive elements superimposed on said center fin for cancelling reflections introduced by said asymmetrically located means, said series elements comprising notches in said counter fin which act as series inductances, and a vertical post on said center fin which acts as a capacitive shunt element.

References Cited in the file of this patent UNITED STATES PATENTS 2,433,368 Johnson et al Dec. 30, 1947 2,574,433 Clapp Nov. 6, 1951 2,623,121 Loveridge Dec. 23, 1952 2,659,005 Gruenberg Nov. 10, 19953 2,659,817 Cutler Nov. 17, 1953 2,735,958 Brown Feb. 21, 1956 2,770,780 Warnecke, et a1 Nov. 13,, 1956 2,799,831 Fubini July 16, 1957 2,806,210 Edwards Sept. 10, 1957 2,829,348 Kostriza Apr. 1, 1958 2,349,711 MacKimmie Aug. 26, 1958 2,903,656 Weisbaum Sept. 8, 1959 OTHER REFERENCES Some New Microwave Antenna Design Based On The Trough Wave Guide, Rotman and Karas, 1956 IRE Convention Record, vol. 4, part 1, copyright 1956, pages 230-235.

The Bell System Technical Journal, vol. 34, No. 1, January 1955, pages 71 and 72, 

